Get rid of the light bulb immediately.
Replace it with a thermostat that is plugged into an undertank heater (or flexwatt heat tape).
Do not use a tank; instead, use a tub.
Make sure you are measuring temperatures with a digital thermometer, such as the acu-rite ones.
This is what I personally recommend for you to have the best and least stressful experience with bp. Also, there are stickies on this website that cover the care of bp more elaborately.